1. Field of the Invention
The present invention relates to a capacitor insulating film, a capacitor, and a semiconductor device.
2. Description of Related Art
Strontium titanate (SrTiO3; hereinafter referred to as “STO”) is well known as a capacitor insulating material having a very high permittivity. For example, Japanese Patent Laid-Open No. 2006-245612 describes a method for manufacturing a capacitor element that uses an STO film as a capacitor insulating film.
However, STO has a band gap of only about 3 eV and is an insulator similar to a semiconductor. Thus, applying a voltage to a capacitor element including an STO film may disadvantageously result in a rapid increase in leakage current from the capacitor element.
For capacitor elements used in semiconductor devices such as DRAMs, the density of a leakage current flowing when an electric field of about 1 MV/cm is applied to the capacitor element needs to be at most 1E-8 A/cm2. A reduction in leakage current is important in making the capacitor element available in recent semiconductor devices for which a reduction in power consumption is demanded.
In recent years, with increasing miniaturization and integration of semiconductor devices such as DRAMs, the size of capacitors forming memory cells has been reduced. This has made provision of a sufficient amount of accumulated charges difficult. To provide a sufficient amount of accumulated charges, the capacitor insulating film needs to have a high permittivity. However, the current design rule for semiconductor devices such as DRAMs specifies at most 50 nm. Thus, even with a capacitor insulating film with a high permittivity, the film thickness of the capacitor insulating film needs to be set to at most 10 nm in order to obtain the desired capacitance of the capacitor. To allow such a reduction in film thickness, an electric field of about 1 MV/cm is inevitably applied between capacitor electrodes. This may disadvantageously result in a high leakage current and even a quick dielectric breakdown in the case of an STO
Toyota Central R&D Labs., Inc. Review, Vol. 32, No. 3 (1997. 9) pp. 61-70 contains data on the characteristics of STO indicating that STO has a high relative permittivity of about 200 but that even an electric field (the intensity of an electric field applied during the flow of a current of 1E-8 A per unit area of 1 cm2) of less than 0.1 MV/cm results in a leakage current density of 1E-8 A/cm2. The results of the inventor's experiments also showed that STO had a relative permittivity of 160 and a breakdown voltage of at most 0.4 MV/cm (under the measurement condition of a leakage current of 1E-8 A/cm2).
Furthermore, Toyota Central R&D Labs., Inc. Review, Vol. 32, No. 3 (1997. 9) pp. 61-70 also describes that an increase in the composition ratio of titanium (Ti) to strontium (Sr) Ti/Sr (Ti rich) improves the relative permittivity and the breakdown voltage; in this case, the relative permittivity is about 140 and the breakdown voltage is 0.8 MV/cm (under the measurement condition of a leakage current of 1E-8 A/cm2). The present inventors have also experimentally confirmed this and obtained data similar to that in this document. However, the breakdown voltage still fails to reach 1 MV/cm.
On the other hand, as a high-permittivity material different from STO, barium strontium titanate ((BaxSr1-x)TiO3; hereinafter referred to as “BST”) is known in which a part of an Sr site of STO is substituted with a barium element (Ba). Substituting a part of the Sr site with Ba results in an increased permittivity. However, the resultant leakage current characteristics are equivalent to those of STO. An example of a document relating to BST is IEICE (The Institute of Electronics, Information and Communication Engineers) Transactions on Electronics, Vol. E77-C, No. 3 (1994) pp. 385-391. The document describes that barium strontium titanate exhibits a relative permittivity of at least 200 but a breakdown voltage of about 0.5 MV/cm (under the measurement condition of a leakage current of 1E-8 A/cm2), which is not much different from that of STO.